Nematic, Smectic C and other liquid crystal devices are routinely used in display applications. They are also used in a variety of other devices, such as variable retarders and laser stabilizers, to control light. Most such devices require alignment layers, that is, layers on the surface of transparent electrodes which cause the special axes of the liquid crystal to align in a specific direction relative to the electrode surface.
The literature concerning liquid crystal displays is vast. The popular book “Liquid Crystals: Nature's Delicate Phase of Matter” by Peter Collings (Princeton Science Library, ©1990) is a readable introduction to the subject and is incorporated herein by reference.
Rubbed polyimides and polyimide-amides are the “standard” material used in the liquid crystal industry today for manufacturing liquid crystal displays. Rubbed nylon 6—6 and rubbed Teflon® are also known and used in liquid crystal research. However, there are few commercial uses in the liquid crystal industry for these types of alignment layers.
Rubbing, more commonly referred to as buffing, is a technique used to align liquid crystal material deposited onto the surface of a liquid crystal display. In practicing this buffing technique, thin coatings of a long chain polymer are applied to the facing surfaces of the two transparent plates between which the liquid crystal layer is disposed. By subsequently rubbing these coatings with a soft material such as cotton cloth or paper, the molecules on the respective coatings can be oriented so that the long axes of the liquid crystal molecules adjacent the respective plates will align parallel to the rubbing direction. This technique, however, has several inherent disadvantages. First, the rubbing operation introduces unwanted contamination onto the polymer coatings because the materials which are used to rub the polymer are generally something other than the polymer itself. For example, cotton or paper are commonly used to rub or buff the polymeric liquid crystals into the desired alignment. Unwanted contaminants from the paper or cotton buffing medium may remain on the liquid crystal surface after the buffing is complete or may contaminate clean-room manufacturing facilities. Second, the rubbing operation introduces considerable shearing forces on the polymer film and may tear the film away from the substrate during manufacture.
Further, buffing generates static electricity which may modify or destroy the underlying active matrix (transistor array) in certain displays. It is also rather ill controlled and depends on a number of poorly understood parameters such as the exact nature of the rubbing cloth, the processing of the polyimide, the humidity of the manufacturing plant, etc. A significant difficulty is that it is not well understood which parameters the buffing does depend on. This results appreciable loss of nearly finished (but inoperative) product and loss of through-put.
Materials modified by exposure to Ultra Violet (UV) light are under development. There are some technical difficulties associated with the UV exposure. Achieving and controlling pre-tilt has not yet been well demonstrated in such systems. The chemical reactivity of the starting materials as well as the reactivity of the final structures is a difficulty. However, patterning the direction of the alignment is more easily accomplished by utilizing the UV method.
Langmuir-Blodgett (L-B) films are also under development. However, there is no clear evidence that this can be done sufficiently quickly and reproducibly for a commercially viable manufacturing process. There are also severe cleanliness issues which are of concern in connection with the production of L-B films.
Obliquely evaporated silicon dioxide films are old, expensive technologies. They require rather good vacuums, slow and expensive evaporators, and are not currently used extensively in industry.
Other processes for producing aligned liquid crystal displays are disclosed in the following U.S. patents:
Harsch (U.S. Pat. No. 3,941,901) disclose a surface alignment method for liquid crystal cells. The method of Harsch comprises applying to the surfaces of transparent plates bounding a liquid crystal film, a long chain polymer such as polyvinyl alcohol or polyvinyl butyral, which is subjected to a shear thinning technique to cause elongation and alignment of these long chain polymers. The polymers used by Harsch are non-rigid, non-ionic polymers.
Omeis et al. (U.S. Pat. No. 5,247,377) relates to a process for producing thin, anisotropic layers composed of liquid crystalline substances. The liquid crystalline substances are applied in a thin layer to one side of a support having a surface restructured in such a way that the structure is given a preferred direction which determines orientation of the liquid crystalline substance and data storage devices produced.
Various compositions are used to prepare liquid crystal displays. For example, Ahne et al. (U.S. Pat. No. 4,619,500) relates to a method for producing orientation layers for liquid crystal displays wherein a solution of an organic prepolymer of polyoxozoles, polythiazoles, polyimidazoles, polyoxazinones, polyoxazine diones or polyquinoxalines is applied to a transparent substrate and subsequently annealed and subjected to an orientation treatment. Such orientation treatments include buffing.
Coates et al. (U.S. Pat. No. 5,426,009) relates to a polymeric composite material which is based on a liquid crystal polymer component. The polymeric composite of Coates exhibits a high glass transition temperature of at least 60° C. and a scattering texture when deposited as a thin film. The polymeric composites of Coates can be rendered transparent by being heated above the glass transition temperature and/or clearing temperature. The polymeric composites of Coates are obtained by mixing a liquid crystal polymer component, a reactive liquid crystalline component, optionally a polymerization initiator component, and/or further additive components, with subsequent polymerization.
However, up to the present time, it has not been known to utilize a rigid-rod poly(ionomer) composition in a buff-free alignment process to produce a liquid crystal display device having planar alignment and pretilt.